The Session Initiation Protocol (SIP) was initially designed to enable Internet users with personal computers or endpoints to set up real-time sessions that exchange voice, video, and/or data for free. However, the usage of SIP so far has been limited within closed IP networks, such as commercial carrier's IP Multimedia Subsystem (IMS), or Next Generation Networks (NGN), or enterprise Voice over IP (VoIP) networks. In addition, calls between enterprise SIP-based VoIP networks are infeasible, since a centralized Internet-wide SIP routing scheme is difficult to implement and maintain.